Virtually all animals have mechanisms to recognize light. Few, however, have eyes.
At the base of any structure that receives light stimuli, however, there are photosensitive pigments that, when hit by radiation of certain wavelengths, undergo energetic changes, transmitting them to sensory cells.
In the medusoid cannons are groups of cells with pigments that simply recognize the existence of light. From there, on the zoological scale, increasingly complex structures are observed.
Whenever there is an eye on the animal scale, its structure is very similar to that of a chalice facing the interior of the body, with the surface covered by the epidermis or cuticle. Any eye works like a camera. On the inner surface of the chalice are sensitive-linked pigmented cells, which carry information to higher centers.
In general, in front of the eye there is a cornea, protective and transparent epithelial membrane. Then there is a muscle, the Iris, which works like a camera diaphragm. The central hole is the pupil. The iris has smooth muscle fibers arranged in a circle and radially. The pupil diameter is regulated by the joint action of these muscles according to the ambient light, thus adjusting the amount of light radiation that hits the sensitive part of the eyeball.
Variation in pupil diameter regulates the amount of light that falls into the eye. Next comes a lens, the lens, attached to muscles that regulate its curvature, which is important for the focusing mechanism of objects. Along with the cornea and fluid in the eye, this lens is the medium through which light passes through the pathway to a layer containing pigmented cells called retina.
In the retina, two types of cells that contain pigments inside the eye catch the eye: cones and the rods. The rods are more abundant in the periphery of the retina and are stimulated with low intensity light. It is often said that they are used for dark vision and do not register color. The cones, in turn, occur mainly in the central region of the retina and their stimulation depends on high light intensities, recognize colors and are said to be cells used when there is clarity.
When pigments are stimulated, they generate energetic modifications, which are transmitted to the sensory cells, whose extensions come together, forming the optic nerve. It connects with the brain, conducting the impulses to a certain area of the occipital lobe, where information is decoded and images are recognized.
Note that at the point where the optic nerve exits towards the brain there are no rods or cones - so at this place there is no imaging, so it is called a blind spot.